Several approaches to metering the video and/or audio tuned by television and/or radio receivers in order to determine the sources or identities of corresponding television or radio programs are known. For example, one approach is to real time correlate a program to which the tuner of a receiver is tuned with each of the programs available to the receiver as derived from an auxiliary tuner. An arrangement adopting this approach is disclosed in U.S. application Ser. No. 08/786,270 filed Jan. 22, 1997. Another arrangement useful for this measurement approach is found in the teachings of Lu et al. in U.S. Pat. No. 5,594,934.
There are several desirable properties for a correlation system. For example, good matches or mismatches should result from very short program segments. Longer program segments delay the correlation process because the time taken to scan through all available programs increases accordingly. Also, the correlation score should be high when the output from the receiver and the output from the auxiliary tuner correspond to the same program. Matches between two different programs must occur very infrequently. Moreover, the matching criteria should be independent of signal level so that signal level does not affect the correlation score.
Another approach is to add ancillary identification codes to television and/or radio programs and to detect and decode the ancillary codes in order to identify the encoded programs or the corresponding sources of the programs when the programs are tuned by monitored receivers. There are many arrangements for adding an ancillary code to a signal in such a way that the added code is not noticed. For example, it is well known to hide such ancillary codes in non-viewable portions of television video by inserting them into either the video's vertical blanking interval or horizontal retrace interval. An exemplary system which hides codes in non-viewable portions of video is referred to as “AMOL” and is taught in U.S. Pat. No. 4,025,851. This system is used by the assignee of this application for monitoring transmissions of television programs as well as the times of such transmissions.
Other known video encoding systems have sought to bury the ancillary code in a portion of a television signal's transmission bandwidth that otherwise carries little signal energy. An example of such a system is disclosed by Dougherty in U.S. Pat. No. 5,629,739, which is assigned to the assignee of the present application.
Other methods and systems add ancillary codes to audio signals for the purpose of identifying the signals and, perhaps, for tracing their courses through signal distribution systems. Such arrangements have the obvious advantage of being applicable not only to television, but also to radio and to pre-recorded music. Moreover, ancillary codes which are added to audio signals may be reproduced in the audio signal output by a speaker. Accordingly, these arrangements offer the possibility of non-intrusively intercepting and decoding the codes with equipment that has a microphone as an input. In particular, these arrangements provide an approach to measuring broadcast audiences by the use of portable metering equipment carried by panelists.
In the field of encoding audio signals for program audience measurement purposes, Crosby, in U.S. Pat. No. 3,845,391, teaches an audio encoding approach in which the code is inserted in a narrow frequency “notch” from which the original audio signal is deleted. The notch is made at a fixed predetermined frequency (e.g., 40 Hz). This approach led to codes that were audible when the original audio signal containing the code was of low intensity.
A series of improvements followed the Crosby patent. Thus, Howard, in U.S. Pat. No. 4,703,476, teaches the use of two separate notch frequencies for the mark and the space portions of a code signal. Kramer, in U.S. Pat. No. 4,931,871 and in U.S. Pat. No. 4,945,412 teaches, inter alia, using a code signal having an amplitude that tracks the amplitude of the audio signal to which the code is added.
Program audience measurement systems in which panelists are expected to carry microphone-equipped audio monitoring devices that can pick up and store inaudible codes transmitted in an audio signal are also known. For example, Aijalla et al., in WO 94/11989 and in U.S. Pat. No. 5,579,124, describe an arrangement in which spread spectrum techniques are used to add a code to an audio signal so that the code is either not perceptible, or can be heard only as low level “static” noise. Also, Jensen et al., in U.S. Pat. No. 5,450,490, teach an arrangement for adding a code at a fixed set of frequencies and using one of two masking signals in order to mask the code frequencies. The choice of masking signal is made on the basis of a frequency analysis of the audio signal to which the code is to be added. Jensen et al. do not teach a coding arrangement in which the code frequencies vary from block to block. The intensity of the code inserted by Jensen et al. is a predetermined fraction of a measured value (e.g., 30 dB down from peak intensity) rather than comprising relative maxima or minima.
Moreover, Preuss et al., in U.S. Pat. No. 5,319,735, teach a multi-band audio encoding arrangement in which a spread spectrum code is inserted in recorded music at a fixed ratio to the input signal intensity (code-to-music ratio) that is preferably 19 dB. Lee et al., in U.S. Pat. No. 5,687,191, teach an audio coding arrangement suitable for use with digitized audio signals in which the code intensity is made to match the input signal by calculating a signal-to-mask ratio in each of several frequency bands and by then inserting the code at an intensity that is a predetermined ratio of the audio input in that band. As reported in this patent, Lee et al. have also described a method of embedding digital information in a digital waveform in pending U.S. application Ser. No. 08/524,132.
U.S. patent application Ser. No. 09/116,397 filed Jul. 16, 1998 discloses a system and method using spectral modulation at selected code frequencies in order to insert a code into the program audio signal. These code frequencies are varied from audio block to audio block, and the spectral modulation may be implemented as amplitude modulation, modulation by frequency swapping, phase modulation, and/or odd/even index modulation.
Yet another approach to metering video and/or audio tuned by televisions and/or radios is to extract a characteristic signature (or a characteristic signature set) from the program selected for viewing and/or listening, and to compare the characteristic signature (or characteristic signature set) with reference signatures (or reference signature sets) collected from known program sources at a reference site. Although the reference site could be the viewer's household, the reference site is usually at a location which is remote from the households of all of the viewers being monitored. The signature approach is taught by Lert and Lu in U.S. Pat. No. 4,677,466 and by Kiewit and Lu in U.S. Pat. No. 4,697,209.
In the signature approaches, audio characteristic signatures are often extracted. Typically, these characteristic signatures are extracted by a unit located at the monitored receiver, sometimes referred to as a site unit. The site unit monitors the audio output of a television or radio receiver either by means of a microphone that picks up the sound from the speakers of the monitored receiver or by means of an output line from the monitored receiver. The site unit extracts and transmits the characteristic signatures to a central household unit, sometimes referred to as a home unit. Each characteristic signature is designed to uniquely characterize the audio signal tuned by the receiver during the time of signature extraction.
Characteristic signatures are typically transmitted from the home unit to a central office where a matching operation is performed between the characteristic signatures and a set of reference signatures extracted at a reference site from all of the audio channels that could have been tuned by the receiver in the household being monitored. A matching score is computed by a matching algorithm and is used to determine the identity of the program to which the monitored receiver was tuned or the program source (such as the broadcaster) of the tuned program.
There are several desirable properties for audio characteristic signatures. The number of bytes in each characteristic signature should be reasonably low such that the storage of a characteristic signature requires a small amount of memory and such that the transmission of a characteristic signature from the home unit to the central office requires a short transmission time. Also, each characteristic signature must be robust such that characteristic signatures extracted from both the output of a microphone and the output lines of the receiver result in substantially identical signature data. Moreover, the correlation between characteristic signatures and reference signatures extracted from the same program should be very high and consequently the correlation between characteristic signatures and reference signatures extracted from different programs should be very low.
Accordingly, the present invention is directed to the extraction of signatures and to a correlation technique having one or more of the properties set out above.